They are also commonly referred to as a "rep" (short for "representation"), and are similar to talent agents in their business model and function.
Most full time working Illustrators are painters or designers who are represented by art agents.
They will often develop relationships with buyers who need to continuously purchase art, like book publishing companies and advertising agencies.
Many professional buyers will only work with an art agent since they can be expected to already understand the business, pricing, and contract negotiation.
Art agents select their artist not based on their talent but the likelihood their works will sell.